The Egg and Bottle Trick.—An ordinary water-bottle, a hard-boiled egg, divested of its shell, and a piece of thin paper are all that is requisite. How can we make this egg get inside the bottle? Light the paper, quickly thrust it into the bottle, and immediately place the egg over the mouth of the bottle, gently pressing it closely down to the glass. The burning paper consumes some of the air, a partial vacuum is formed, and air pressure will force the egg into the bottle with a loud detonation. (See Fig. 17.)
Fig. 17.
Making Water Boil by Means of Coldness.—Heat some water to boiling in a glass flask over a spirit lamp. After the water has boiled for a minute or two, quickly insert a well-fitting cork, and remove the flask from the flame. Wrap a duster or towel round the neck of the flask, and, holding it over a basin (in case of breakage), pour gently a stream of cold water on to the flask. The steam inside is condensed, a partial vacuum is formed, and as long as any heat remains in the water, it will boil, whilst the stream of cold water is continued on the outside. When ebullition no longer occurs, it will be found that the cork is held in so tightly by air pressure that it is very difficult to draw it. (See Fig. 18.)
Fig. 18.
Fire Designs.—This is very simple, amusing, and effective. Make a saturated solution of nitrate of potash (common nitre or saltpetre), by dissolving the substance in warm water, until no more will dissolve; then draw with a smooth stick of wood any design or wording on sheets of white tissue paper, let it thoroughly dry, and the drawing will become invisible. By means of a spark from a smouldering match ignite the potassium nitrate at any part of the drawing, first laying the paper on a plate or tray in a darkened room. The fire will smoulder along the line of the invisible drawing until the design is complete. (See Fig. 19.)
Fig. 19.